One known method to build a radio signal direction-finding system includes creating a single antenna with a narrow beamwidth. When the signal received by the antenna is relatively strong, then the radio source is known to be in the direction the antenna's beam is pointed to. A narrow beam is necessary to obtain the source's direction with good angular resolution. It is noted that an antenna's beamwidth is inversely related to its size. An antenna with a sufficiently narrow beam tends to be inconveniently large.
Another known method of determining the direction of arrival of a radio signal is a sequential lobing technique. In the sequential lobing technique, an antenna system is deployed which has two beams (also known as lobes) pointing in two different but nearby directions. This technique typically involves activation of the two beams alternately and sequentially on sequential radio transmissions or radar pulses. The signal strengths from the two beams can be compared to each other. In the sequential lobing technique, the source of the radio signal is deduced to be in the direction of the stronger beam and the ratio of the strengths indicates the degree the radio signal is directed toward the stronger beam. The sequential lobing technique does not require especially narrow beam widths and thus the antenna(s) can be relatively small.
However, transmitter instability, changing multipath and other variations can cause the received signal strength to change from the activation of one beam to the other for reasons unrelated to direction of arrival. A radio signal direction-finding system based on the sequential lobing technique may misinterpret these changes as the direction of arrival indication.